1. Technical Field
The present invention relates to completing a subterranean formation and in one of its aspects relates to a method and apparatus for fracturing different levels of a completion interval of a subterranean formation(s) in a single operation.
2. Background Art
In completing wells used in the production of hydrocarbons or the like, it is common to xe2x80x9chydraulically fracturexe2x80x9d at least some interval of the production/injection formation(s) in order to improve flow of fluids into and/or out of the formation. As is well understood in the art, hydraulic fracturing is typically carried out by lowering a workstring into the well and isolating that portion of the wellbore which lies adjacent the interval to be fractured by setting packers or the like. Fracturing fluid or slurry (e.g. a heavy gel with or without props) is then pumped down the workstring and into the isolated interval at a pressure sufficient to separate or xe2x80x9cfracturexe2x80x9d the formation, thereby forming permeable channels within the formation.
In thin or relatively short intervals that are fairly homogeneous, standard fracturing techniques such as described above will normally produce a fracture or fractures throughout the length of the completion interval. However, these standard fracturing techniques experience problems when used in fracturing long or thick intervals or in intervals which are heterogeneous (i.e. made of several levels or zones which fracture under different pressures). For example, it is difficult, if possible at all, to fracture a second zone in such intervals once a first zone has started to fracture. The fracturing slurry will continue to flow into and enlarge the initial fracture as the pressure increases in the isolated portion of the wellbore rather than initiate additional fractures in the other zones or levels of the fracture interval.
Further, liquid from the fracturing slurry is typically xe2x80x9clostxe2x80x9d into the formation through the initial fracture causing props, e.g. sand, to settle out of the slurry thereby forming a bridge or blockage within the wellbore adjacent the initial fracture. Such blockages prevent further flow of slurry to other zones in the fracture interval even if such zones had already experienced some initial breakdown, i.e. fracturing. This results in poor distribution of fractures throughout the fracture interval since often only the zone having the lowest breakdown pressure will be adequately fractured and propped.
Due to these problems, it is common to fracture a long and/or heterogeneous interval by carrying out a series of individual, conventional fracturing operations such as described above. That is, a first zone is fractured, then the workstring is repositioned in the wellbore and a second zone is fractured, and so on until the entire interval has been fractured. Of course, as will be recognized by those skilled in this art, this repetition is both expensive and time consuming and can significantly affect the overall economics of the well being completed.
To overcome the problems in fracturing long and/or heterogeneous intervals, several methods have been proposed wherein the fracturing of such intervals can be carried out with a single setting of a workstring wherein the fracturing slurry is delivered simultaneously to the different levels or zones within the interval through alternate flowpaths; for example, U.S. Pat. No. 5,161,618 to Jones et al. Another such method is disclosed in U.S. Pat. No. 5,435,391, issued Jul. 25, 1995, wherein alternating slugs of a gel and a proppant slurry are pumped down a single workstring and through alternate flowpaths to fracture and prop different levels within the fracture interval.
Still another such method is that disclosed in U.S. Pat. No. 5,417,284, issued May 23, 1995, wherein a fracturing gel is pumped down a workstring and into one end of the isolated wellbore while a proppant slurry is pumped at the same time through the well annulus and into the other end of the isolated annulus to carry out the fracturing of the different levels within the isolated interval. If a blockage occurs, the fracturing gel and/or slurry continues to be delivered throughout the fracture interval through alternate flowpaths to complete the fracturing of the interval.
A similar method is disclosed in U.S. Pat. No. 5,560,427 except a slurry splitter is positioned downhole in the workstring wherein a portion of the gel is separated from the fracturing slurry and is flowed into the bottom of the isolated interval to initiate fracturing within the interval. The remainder of the slurry is flowed into the upper end of the isolated interval to prop the fractures as they are being formed. Alternate flowpaths are provided to insure that the gel and/or slurry will be delivered to the different levels within the interval should a blockage occur in the well annulus before the fracturing operation is complete.
The present invention provides a method and apparatus for fracturing different levels of a completion interval of a subterranean formation which is traversed by a wellbore of a well. Basically, a workstring comprised of a tubing having a cross-over and a fracturing string is lowered into the wellbore until the fracturing string is positioned adjacent the completion interval to be fractured. The fracturing string is comprised of a base pipe which is essentially blank over most of its length (i.e. impermeable) except for a plurality of perforated sections which are spaced along its length. Each perforated section is formed by a plurality of openings (e.g. round holes, slots, etc.) through the wall of the base pipe which are spaced around and throughout the length of the perforated section. The lengths of the respective perforated sections may vary (e.g. from about 1 to about 300 feet) with the perforated sections being spaced from each other along the base pipe at varying distances (e.g. from about 10 to about 1000 feet).
In fracturing formations where little or no substantial particulate material, e.g. sand, will be produced with the formation fluids, the openings in the perforated sections may be merely un-screened openings through the base pipe, similar to the slots in a conventional xe2x80x9cslotted linerxe2x80x9d. In formations where substantial sand will be produced, a screen means, e.g. wire wrap, is positioned over the openings at each perforated section to allow fluid to flow into the base pipe through the openings while preventing any substantial sand from flowing therethrough. At least one alternate flow path (e.g. shunt tubes) having an inlet and one or more spaced outlets, is provided on the fracturing string and extends longitudinally along the length of the base pipe. The wire wrap can be wrapped over the shunt tubes at each perforated section or the shunt tubes can be bent to pass over the wire wrap after the wire is in place on the base pipe. Where the shunt tubes are outside the wire wrap, a perforated sleeve or shroud can be positioned over the shunts at each perforated section to protect the shunt tubes during installation.
To carry out the fracturing method of the present invention, the workstring is positioned within the wellbore so that the fracturing string will extend substantially through said completion interval and will form a xe2x80x9ccompletion interval annulusxe2x80x9d with the wellbore which, in turn, is isolated from the well annulus above. A fracturing slurry, comprised of a fracturing liquid (e.g. high-viscosity gel) and proppants (e.g. sand), is flowed down the tubing string and out through the cross-over into the top of the completion interval annulus. As is common in routine fracture operations, a pad of fracturing fluid without proppant can be pumped into the completion interval annulus before the slurry to initiate the fracturing of the formation, if desired.
As will be understood in the art, when returns are taken through the well annulus, liquid from the fracturing slurry is lost both into the formation(s) within said completion interval and into said base pipe through the openings in the perforated sections along the base pipe. This causes the proppants (sand) from the slurry to xe2x80x9csand-outxe2x80x9d within the completion interval annulus at the perforated sections to form sand bridges which, in turn, block further flow of slurry down through the completion interval annulus.
These sand bridges or blockages effectively act as packers which isolate portions of the completion annulus which lie between respective adjacent, perforated sections. The pumping of the slurry into the top of the completion interval annulus is continued but now it can only flow downward therein through the alternate flowpaths, i.e. shunts tubes. The slurry enters the tops of the tubes and flows downward to exit through the spaced outlets at different levels within the completion interval annulus; that is, the slurry exits into the isolated portions of the completion interval annulus. Continued pumping of the slurry will cause the pressure to build up within these isolated sections until the different levels within the completion interval are fractured and propped with the proppant. If any level of the completion interval is not to be fractured, no outlets are provided in the shunt tubes at this level; accordingly, no fracturing slurry can exit into the isolated portion of the completion interval annulus which lies adjacent the level which is not to be fractured.
Where returns are not to be taken through the well annulus, the flow of fluid into or through the base pipe is blocked since the base pipe and well annulus are filled with a non-compressible, completion fluid. Therefore, no substantial amount of liquid from the fracturing slurry will be initially lost through any of the perforated sections but will only be lost into the formation. Eventually, the formation will fracture at some level within the completion interval. Once this initial fracture is formed, liquid can now flow not only into the fracture but also into the base pipe through some of the perforated sections and back out into the completion interval annulus through the perforated section nearest the initial fracture.
This causes sand bridges to form at the perforated sections where liquid is being lost from the slurry. These sand bridges form xe2x80x9cpackersxe2x80x9d which, in turn, isolate the portions of the completion interval annulus which lie therebetween. Slurry now can only flow through the alternate flow paths which deliver the slurry into the isolated portions of the completion interval annulus to finish the fracturing operation.
Once the completion interval has been fractured and propped, flow of slurry is ceased and the well is put on production. The fluids from the completion interval flow into the completion interval annulus and due to the difference in the viscosity of the slurry liquid (e.g. about 100 centipoises) and the produced fluids (e.g. about 1 centipoise), the produced fluids can readily flow through the sand bridges and into the base pipe through the openings in the perforated sections of the base pipe. Where a substantial amount of sand is produced with the formation fluids, the openings in the perforated section will be equipped with a screen means (e.g. wire wrap) which allows the produced fluids to pass therethrough while blocking any substantial flow of particulates.